I know this is a by design. However, it is very important to enable code to realize whether the drop is allowed, and whether it should be copy or move when the user is dragging over an element.
This is not a JSC bug. Events? Webcore bindings? - not sure, punting in the right approximate direction!
I think that this is a duplicate of https://bugs.webkit.org/show_bug.cgi?id=23695
Not a dupe, as this appears to be a separate request to change the design of the spec in favor of a more flexible security policy.